This issue aims these paragraphs fromhttps://symfony.com/doc/current/mailer.html#always-send-to-the-same-address :

Use theallowed_recipients option to specifyexceptions to the behavior defined
in therecipients option; allowing emails directed to these specific recipients
to maintain their original destination:

With this configuration, all emails will be sent toyouremail@example.com,
except for those sent tointernal@example.com,internal-monitoring@example.fr,
etc., which will receive emails as usual.

The wording "exceptions" and "except" is a bit misleading, to me it means that the mail is sent toallowed_recipients addresses and notrecipients addresses anymore. But in fact, the mail is still sent torecipients addresses.

It's not a feature bug as the contributor intended toreplicate the whitelist behaviour of swiftmailer, but it'sexplicitly mentioned in swiftmailer documentation that the mail is still sent torecipients:

all email addresses matching these regexes will be delivered, like normal, as well as being sent todev@example.com

Linked issues :#19769 &#19775